An electric vehicle (EV), plug-in hybrid electric vehicle (PHEV), or hybrid electric vehicle (HEV) may be provided with a power circuit that may include multiple high voltage power relays. The high voltage power relays, which may be arranged in an EV/PHEV/HEV junction box, may be operable to conduct electrical power from a vehicle power source to a vehicle system or subsystem. A controller may be in communication with the power circuit to command the relays open or closed.
A system and method is usually required to effectively monitor relay contact state (i.e., determine whether a relay contact is open or closed) and provide a reliable diagnosis for welded relay contacts in junction boxes of electric vehicles. More generally, such a system and method may be required to diagnose electrical continuity or electrical interruption of any conducting path, such as in relay contacts, fuses or busbars.
Usually, voltage measurements at both sides of a relay contact are used for diagnosis of the state of the main contactors of the relay. This strategy, involving a direct current (DC) measurement with duplication of sensing channels, fails to detect the case where no current is flowing through the contact since no voltage drop is developed. This strategy also forces the sensing channel and electronics to withstand high DC common mode negative voltages when contacts are open. Therefore, a need exists for a system and method to effectively detect relay contact state independently of current flowing through said contacts and that does not suffer from negative common mode voltages.